The present invention relates to polyacetal compositions that are especially useful in plastic-plastic wear systems. A wear system is one in which the surface of one part of the system bears against the surface of another part of the system, thereby causing friction and wear of each part. In a plastic-plastic system, the parts of the system bearing against each other are each plastic. In addition, it was found that the compositions of the present invention have good warpage properties.
More specifically, the present invention is directed towards the use of polyacetal compositions containing ethylene methyl acrylate and a lubricant, and optionally polytetrafluoroethylene fibers, in said wear systems. The compositions are especially useful in wear systems due not only to their excellent wear resistance, but also due to their ability to maintain substantially constant wear resistance under varying maximum pressure molding conditions. Further, it has been found that the compositions of the present invention that contain pigments, said pigments often being added in the final applications, also maintain substantially constant wear in the face of varying molding conditions, especially maximum pressure molding conditions.
It is known that polyacetal compositions are used in plastic-plastic wear systems, such as, in particular, conveyor chain systems. Conveyor chains are well known in the art. Generally, they are made from a series of links having flat tops, said links being connected to each other by pins. The pins are in most cases made from stainless steel or plastic resins. The conveyor chain links can be made from thermoplastic resin, such as polyacetal resin. The conveyor chain links move on a guided rail and they are driven by gear sprockets located on both ends of the conveyor chain line. The gears can also be made of polyacetal resin.
There are many types of conveyor chains. However, two types of conveyor chains, the flat-top conveyor chain and the open hinge conveyor chain, are predominant.
Flat top conveyor chains move straightly and flexibly. They are generally made of single width chain links having relatively limited contact with the driving gear sprockets. However, these chain links have noticeable contact with guided rails, said rails usually being made of stainless steel or wear resistant plastic resin. Flat top conveyor chain links are connected to each other by pins, said pins also usually being made of stainless steel or wear resistant plastics. Thus, in a flat top conveyor chain system, wear occurs between the pin and the chain link, on the flat top surface due to carried materials, on the part of the link in contact with the driving sprockets, on the part of the link in contact with the guide rails, and on the part of a link that is in contact with another link. In order to improve, or lessen, the degree of wear experienced by links in such a conveyor chain system, the mold design of the conveyor chain link is becoming more and more complex. Further, many end-users prefer to add pigments to the compositions used for making the chain components. As such, there also exists a need to develop pigmented compositions that are not unduly influenced by variations in molding conditions.
Open hinge conveyor chains have a wide carrying table and a large contact area between the chain links and the drive sprockets. Open hinge conveyor chain links have open areas constituting from 10% to 50% of the link, depending on the design. Open hinge conveyor chains are mostly used for straightly transporting foods. As such, it is desirable for the links to have these open areas because food is often cleaned directly on the links and also it permits easy cleaning of the chain links themselves. However, these open areas on a chain link create very complicated molding designs.
As the mold designs for conveyor chain links become more and more complicated and color becomes important, it becomes more and more challenging to find suitable thermoplastic resin compositions that will flow sufficiently and economically through the complicated mold design, that will have good resistance to constant wear, and that will not be unduly influenced by varying molding conditions or the inclusion therein of pigments. For example, it has been found that small changes in mold processing parameters (such as molding temperature, injection speed, injection pressure, or molding cycle) and/or small changes in mold gate design (such as the number of gates or the position of the gates) can have great influence on the surface appearance, and in turn, the wear resistance, of molded conveyor chain links. Changes in molding conditions are common in commercial applications. The surface appearance (such as gloss, roughness, and weld line appearance) of a chain link can have an effect on the wear resistance of the chain link. For example, the amount and position of weld lines of the surface of the molded part, or the percentage of solid additives in the resin that are located near the surface of the molded part, can adversely impact the wear resistance of the molded part. As such, there exists a continuing need to develop resins for use in wear applications that have good and substantially constant surface appearances and wear resistances, even though the conditions under which molding occurs are varied and/or pigments are added therein. In addition, there exists a need to develop resins for use in wear applications wherein said resins have improved resistance to warpage.
U.S. Pat. No. 4,436,200 discloses a conveyor chain system wherein the conveyor chain links are molded from a composition of polyacetal and polytetrafluoroethylene fibers. The chain links are reported to have good wear resistance. However, it has been found that although such compositions result in conveyor chain links having good wear resistance, the surface appearance of chain links molded from the composition is strongly influenced by variations in molding conditions (such as temperature, pressure, speed, and/or cycle time) and pigments. This finding is partly due to the fact that while polyacetal compositions containing polytetrafluoroethylene fibers have better wear resistance than polyacetal compositions containing polytetrafluoroethylene micropowder, the fibers are large (e.g., on average, more than 10 micron diameter and 30 micron length), especially when compared to micropowder. The combination of both these factors, it is believed, results in resin that is sensitive to changes in molding conditions and the addition thereto of pigments. Hence, it has been found that the wear resistance of links molded from such a composition varies from one molding condition to another and it varies from the exclusion or inclusion of pigments. As such, there still exists a need for a resin useful for wear applications that has good wear resistance and in addition, maintains that wear resistance over varying molding conditions.
It was unexpectedly found that the incorporation of ethylene methyl acrylate copolymer, along with at least one lubricant that is fluidized below the temperature at which polyacetal is melt processed, into polyacetal results in compositions having good wear resistance. It has further been found that the wear resistance of parts molded from the composition remains substantially constant over varying maximum pressure molding conditions and that it is not unduly influenced by the inclusion therein of pigments. It has further been found that the warpage experienced by parts molded from such resins is less than would be predicted from the individual components. Shaped articles made from these compositions are useful in plastic-plastic wear applications, such as, in particular, conveyor chain link applications, where good and constant wear resistance is required even under varying molding conditions and even with the addition of pigments.